Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect the gas valve harness connector from the pcb
(R/H side, back connector, black, brown and blue wires)

Measurement:

Test 1:
Measure the resistance across brown and blue wires (EV1)
at the disconnected gas valve harness connector.
Resistance should be approximately 870 ohms.
Test 2:
Measure the resistance across black and blue wires (EV2).
Resistance should be approximately 870 ohms.
Test 3:
Measure the resistance across blue wire and a suitable earth.
Resistance should be greater than 1M Ohm.